I would like to second the request for a preloading feature. The reading experience with certain types of heavy PDFs, the already mentioned scanned documents, is subpar.
Ideally, you would add a setting to the performance preferences where the user can configure preloading to arbitrarily high values.
I myself, and I can imagine many others, use the program not just for editing PDFs but also for viewing.
And I'm not talking novels, which are read in a linear fashion. With novels it suffices to preload the next couple of pages. That's true.
I'm talking about non-fiction books, specialist literature, technical documentation etc. With these types of files, one jumps around constantly from position to position. One flips through half the book back and forth to find the relevant passage.
It's immensely deterring, if you have to wait half a second to a second for every page to render.
That the cached images are only usable for a specific zoom ratio is a nonissue. If I read a document, I set the viewmode once to "fit page" and don't fiddle with the zoom. This way my eyes can scan the whole page in a split second. Less time than it takes to render that friggin page. Just like David P. does. Unsurprisingly, because that usage pattern comes naturally for everyone who reads a pdf file the way a reference book is read.
I have 16GB of RAM, a common size for a modern pc. Workstations often have 64GB or more. And often reading that PDF is the single task at hand. It's super frustrating to see all that RAM go unused just to endure those rendering delays.
I'm not intending to be all negative. All in all, PDF-XChange Editor is a tremendous product, that I enjoy using very much. It's just, that the described scenario is highly common for my usage.
Implementing this feature would be a game changer for me and make an already great product truly perfect.
Keep up the good work!